1. Field of the Invention
The present invention relates to methods and aqueous acid compositions for reducing metal surface corrosion and the precipitation of metal sulfides from the aqueous acid compositions when acidizing a sour well or other similar location.
2. Description of the Prior Art
Aqueous acid solutions are commonly used to treat oil or gas wells. For example, subterranean well formations are often contacted with aqueous acid solutions to increase the permeabilities of the formations whereby the production of oil and/or gas therefrom is increased. Aqueous acid solutions are also utilized to etch flow channels in the faces of fractures formed in the formations and to clean out perforations and tubular goods in wells.
When the well being treated is sour, i.e., the fluids produced contain a significant quantity of sulfide compounds, and particularly hydrogen sulfide, problems are often encountered as a result of the precipitation of metal sulfide such as iron sulfide from the aqueous acid solution. That is, dissolved sulfide ions and dissolved ferrous ions from ferrous sulfide scale and/or metal ions from the subterranean formation being treated react to form metal sulfides in the acid solution. The metal sulfides precipitate from the acid solution when the pH reaches a level greater than about 1.9. The pH of spent aqueous acid solutions used to treat subterranean formations is greater than 1.9, and therefore, metal sulfides dissolved in an unspent acid solution precipitate at some point before becoming spent. The metal sulfide precipitates can plug the treated formation and cause serious damage to the well.
Aqueous acid solutions are also used to remove scale from tubular apparatus such as heat exchangers, boilers and the like. If the scale contains metal and sulfide compounds, metal sulfide precipitates can form which interfere with the cleaning process.
While a variety of methods and compositions have been developed heretofore for preventing the precipitation of metal sulfides from aqueous acid treating solutions, there is a continuing need for improved acidizing methods and compositions which are relatively inexpensive and produce little or no metal sulfide precipitates.
The present invention provides methods and aqueous acid compositions for treating subterranean formations in sour wells, tubular apparatus and the like wherein corrosion of metal surfaces contacted by the aqueous acid compositions and metal sulfide precipitation from the aqueous acid compositions are reduced or prevented.
The methods of the present invention for acidizing a sour well containing metal compounds and sulfide compounds are basically comprised of combining an aldol-amine adduct with an aqueous acid solution to preferentially react with sulfide ions subsequently dissolved by the aqueous acid solution and thereby prevent subsequently dissolved metal ions from reacting with the sulfide ions and precipitating. Thereafter, the aqueous acid solution containing the aldol-amine adduct is introduced into the well.
The aqueous acid compositions of the invention are basically comprised of water, an acid selected from the group consisting of inorganic acids, organic acids and mixtures thereof and an aldol-amine adduct which is soluble and stable in the water and acid. The aldol-amine adduct preferentially reacts with sulfide ions when they are dissolved in the acid compositions thereby preventing the dissolved sulfide ions from reacting with dissolved metal ions and precipitating.
It is, therefore, a general object of the present invention to provide improved methods and compositions for reducing metal corrosion and metal sulfide precipitation when acidizing sour wells and the like.
Other and further objects, features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of preferred embodiments which follows.